This is the first part of the RFC, which implements the initial
infrastructure used for capturing expression results.
To capture the expression, it also adds a new annotation token called
annot_input_end, which works somewhat like the annot_end_of_module token
we already have. The idea is proposed by Richard Smith, thanks!
RFC: https://discourse.llvm.org/t/rfc-handle-execution-results-in-clang-repl/68493
Signed-off-by: Jun Zhang <jun@junz.org>